Mesoporous Assembled Mn3O4 Nanoparticle Networks as Efficient Catalysts for Selective Oxidation of Alkenes and Aryl Alkanes

Article abstract of DOI:10.1002/cplu.201600460

The design of nanoscale materials has been considered important for enhancing their surface properties for catalysis. Metal oxide nanoparticles have a large number of exposed surface active sites, but they suffer from low reactivity and poor stability resulting from excessive aggregation into less active microscopic structures. Herein, the synthesis of mesoporous Mn₃O₄ nanoparticle assemblies by polymer-assisted self-assembly is presented and their catalytic activity is demonstrated in the oxidation of various saturated and unsaturated hydrocarbons, including aromatic alkenes and aryl alkanes, in the presence of tert-butyl hydroperoxide as a mild oxidant. It is also shown through comparative studies that the high catalytic activity and stability of these Mn₃O₄ assemblies arise from the unique three-dimensional open-pore structure, high internal surface area (90 m² g^?1) and uniform mesopores (≈6.6 nm in size).

Full text of DOI:10.1002/cplu.201600460

Accepted Article
Title: Mesoporous Assembled Mn3O4 Nanoparticle Networks as Efficient Catalysts
for Selective Oxidation of Alkenes and Aryl Alkanes
Authors: Euaggelia Skliri; Stelios Papadogiorgakis; Ioannis Lykakis; Gerasimos S.
Armatas
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ChemPlusChem
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Mesoporous Assembled Mn O Nanoparticle Networks as
3
4
Efficient Catalysts for Selective Oxidation of Alkenes and Aryl
Alkanes
[
a]
[a]
[b]
[a]
Euaggelia Skliri, Stelios Papadogiorgakis, Ioannis N. Lykakis, and Gerasimos S. Armatas*
Abstract: Designing materials at the nanoscale has been
considered important for enhancing surface properties for catalysis.
Metal oxide nanoparticles have a large number of exposed surface
active sites, but they suffer from low reactivity and poor stability
resulted from the excessive aggregation in less active microscopic
high associated product yield still remains a challenge.
Manganese oxides, especially Mn have attracted
particular attention in catalysis because of their intriguing
3
4
O ,
oxidation–reduction (redox) properties and excellent chemical
stability. As a result, in recent years, a number of Mn
3 4
O -
structures. Herein, we present the synthesis of mesoporous Mn
3 4
O
containing materials have been successfully used in various
[
7]
nanoparticle assemblies by polymer-assisted self-assembly
a
catalytic applications, including degradation of phenols ,
[
8]
[9]
approach and demonstrate their catalytic activity in oxidation of
various saturated and unsaturated hydrocarbons, including aromatic
alkenes and aryl alkanes, in the presence of tert-butyl hydroperoxide
as mild oxidant. We also show, through comparative studies, that the
reduction of nitrobenzenes , and oxidation of arenes ,
[
10]
[11]
alcohols
and alkyl-substituted alkenes
.
In addition,
hausmannite Mn
3
O
4
has received a considerable interest as
electrode material for lithium-ion batteries and supercapacitors
owing to its low cost, good electrochemical performance, and
most impotently excellent cycling stability even at high current
high catalytic activity and stability of these Mn
3 4
O assemblies arise
from the unique three-dimensional open-pore structure, high internal
2
-1
[12]
surface area (90 m g ) and uniform mesopores (ca. 6.6 nm in size).
rates.
3 4
Earlier studies have shown that Mn O nanostructures with a
large number of exposed catalytically active edge sites, such as
nanoparticles, nanorods and nanosheets, are particularly
effective for a variety of redox reactions. For example, these
materials displayed exemplary activity in oxidation of volatile
Introduction
Olefinic hydrocarbons, which are main constituents of oil and
natural gas, are excellent feedstocks for chemical industry.
Catalytic oxo-functionalization of aromatic and aliphatic alkenes
towards the production of more valuable products, such as
alcohols, ketones, acids and peroxides, is one of the most
[13]
organic compounds (VOCs) and electrocatalytic reduction of
[14]
oxygen
.
In principle, small-sized Mn
(1D) nanorods might
3
O
4
NPs and one-
dimensional
exhibit distinctive
functionalities owing to the high reactivity, large exposed surface
area and adequate interactions between the active sites and
[
1]
important transformations in organic chemistry. In this regard,
significant advancements have been made in the use of solid,
recyclable catalysts, in respect of environmental friendly
conditions using molecular oxygen or hydrogen peroxides as
reactants, portending great potentials for use in future catalytic
[7a,15]
applications.
However, despite these promising advances,
thermodynamics favours the excessive agglomeration of such
nanomaterials in large close-packed superstructures (e.g. bulk-
like structures and densely packed NP aggregates) with irregular
shape and low porosity, limiting the catalytic activity.
Consequently, the integration of these nanoscale building blocks
into high surface-to-volume ratio architectures with internal
[
2]
mild oxidants.
The use of solid catalysts offers several
advantages over homogeneous systems, including catalyst
recovery and controlled regio- and chemo-selective
[3]
transformations. So far, although a diverse range of supported
transition-metal oxides, such as CuO, ZnO, CeO , Fe and
2
2
O
3
[5]
[16]
porosity is a promising approach to address these limitations.
[4]
WO
polyoxometalate
Mn ZnW(ZnW
applied to these reactions, the selective oxidation of olefins with
3
,
metal nanoparticles (NPs), such as Pd and Ru, and
We recently demonstrated the synthesis of ordered
4
-
clusters,
and [XW12
such
as
[W10O32] ,
mesoporous iron oxide architectures using ligand-stripped -
1
0-
n-
[6]
[
2
9
O
34
)
2
]
O
40
]
(X = P, Si), have been
[17]
Fe
2
O
3
NPs as primary building blocks.
To construct these
materials, we first utilize nitrosonium tetrafluoroborate (NOBF
4
)
to gently strip the native ligands (oleyl alcohol) from the surface
–
2 3 4
of -Fe O NPs and charge stabilized them by BF anions. This
[
[
a]
b]
E. Skliri, S. Papadogiorgakis, Prof. Dr. G.S. Armatas
Department of Materials Science and Technology
University of Crete
Heraklion 71003 (Greece)
E-mail: garmatas@materials.uoc.gr
Prof. Dr. I.N. Lykakis
Department of Chemistry
Aristotle University of Thessaloniki
Thessaloniki 54124 (Greece)
procedure was necessary to obtain ligand-free NPs that
preferentially segregate to the polar domains of the polymer
template
interactions) during synthesis. Thermal annealing is then used to
decompose the polymer and fuse the NPs into rigid
mesoporous network. As a result of the accessible pore
channels and large surface area, mesoporous -Fe NP
(through
electrostatic
or
hydrogen-bonding
a
2
O
3
assemblies have been shown to be highly effective for reduction
Supporting information for this article is given via a link at the end of
the document.
of nitroarenes into the corresponding aryl amines and oximes,
[
17a]
using methylhydrazine or hydrazine as reducing agents.
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In this study, we report the synthesis of a new mesoporous
network consisting of interconnected Mn O NPs through a
3 4
polymer-assisted self-assembly method and demonstrate its
high activity for the catalytic oxo-functionalization of aliphatic and
aromatic alkenes as well as aryl alkanes using tert-butyl
hydroperoxide as a mild oxidant. An advantage of this synthetic
route is that it allows the construction of extended three-
dimensional (3D) porous networks from metal oxide NPs as
building blocks in contrast to the preparation of isolated NPs and
[
18]
3 4
Scheme 1. Schematic illustration of the synthesis of Mn O MNAs (ASA:
Aggregating Self-Assembly, Cal: Calcination, MNAs: Mesoporous
Nanoparticle Assemblies).
nanorods and close-packed thin films. The resulting material
possesses a network structure of interconnected 6–7 nm-sized
Mn
narrow pore-size distribution. We show that, compared to other
morphologies of Mn such as bulk microparticles, individual
NPs and random NP aggregates, mesoporous assemblies of
Mn NPs have the potential to provide more exposed active
3 4
O NPs and shows highly accessible surface area and
3
O
4
The porous structure of the as-obtained Mn
material was characterized by small-angle X-ray scattering
SAXS), transmission electron microscopy (TEM), X-ray
diffraction (XRD) and N physisorption. The SAXS pattern is
3 4
O MNAs
3
O
4
(
sites between the NPs, leading to improved catalytic
performance. Furthermore, we demonstrate that although these
-catalyzed reactions proceed under mild conditions, they
are particularly selective towards saturated and unsaturated
carbonyl compounds.
2
shown in Figure 1, where a relative broad but resolved
Mn
3
O
4
-1
scattering peak at 0.49 nm q (=4π·sinθ/λ, where 2θ is the
scattering angle and λ is the wavelength of Cu irradiation) range
can be observed. The peak position of this reflection
corroborates to the NP aggregates with an average interparticle
spacing (from d = 2π/q) of 12.8 nm. Moreover, analysis of the
Results and Discussion
[19]
scattering data by the Guinier equation
showed that the
constituting NPs have an average particle size of about 6.9 nm
Figure 1a, inset). As we will show below, this particle size is
very close to the diameter of the starting materials (ca. 7 nm, as
determined from XRD data), which implies minimal grain
Catalysts synthesis and characterization
(
A schematic overview of mesoporous Mn
Mn MNAs) synthesis by the polymer-templated aggregating
self-assembly (ASA) method is shown in Scheme 1. Briefly, for
the synthesis of Mn MNAs, a dimethylformamide (DMF)
solution containing BF -coated Mn NPs was added to an
3 4
O NP assemblies
(
3 4
O
3 4
coarsening of Mn O NPs during synthesis.
3
O
4
–
4
3 4
O
ethanol solution of poly(ethylene oxide)-b-poly(propylene oxide)-
b-poly(ethylene oxide) (EO20PO70EO20) block copolymer. The
reaction mixture was then placed in an oven to boost the
aggregation process. We observed that the initial concentration
of Mn O NPs and the evaporation time of solvents have a
3 4
considerable influence on the porosity of the products. We
obtained optimal results (on the basis of surface area of the final
products) with a slow evaporation of solvents (within ~6–7 days,
o
at 40 C) and using a copolymer:Mn
3
O
4
:ethanol:DMF molar ratio
of 0.07:1:49.7:12.3. In these conditions, as the reaction
progresses, the Mn NPs co-assemble with the liquid-
3
O
4
crystalline block copolymer, and eventually formed a hybrid
polymer–NP supramolecular mesostructure. This intermediate
o
product was then calcined in air at 350 C for 4 h to yield a
3 4
Mn O NP-linked mesoporous network with 3D interconnected
porosity. The complete elimination of the polymer template was
confirmed by thermogravimetric analysis (TGA), where the
mesoporous sample did not show any appreciable weight loss
Figure 1. SAXS pattern of the mesoporous assemblies from Mn
3
O
4
NPs.
2
2
Inset: Guinier plot [ln(I(q))  q ·R
g
/3, where R
g
is the radius of gyration (qR <
g
o
1)] for the Mn O4 MNAs sample derived from the low q-region of the scattering
3
up to 600 C (Figure S1 in the Supporting Information).
curve. The red line is fit to the data.
3 4
Typical TEM images of the Mn O MNAs are shown in
Figures 2a and 2b. The images reveal a disordered porous
network that is constructed solely by individual NPs with grain
sizes 6.7 ±0.9 nm (see the inset in Figure 2a), in agreement with
SAXS results. To probe the crystal structure of the mesoporous
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and c) SAED pattern for the Mn
mesoporous Mn assemblies (Mn
Mn NPs (ii).
3
O
4
MNAs sample. d) XRD patterns of
framework, we utilized high-resolution TEM (HRTEM) and
selected-area electron diffraction (SAED). The HRTEM image, in
Figure 2b (inset), displays a portion of the porous structure and
demonstrates the high crystallinity of constituting NPs; it shows
distinct lattice fringes with 2.9 and 4.9 Å interplanar spacings,
which correspond to the (200) and (101) crystal planes (along
–
O
3 4
3
O
4
MNAs) (i) and isolated BF
4
-capped
3 4
O
The porosity of the template-free material was examined by
nitrogen physisorption measurements. Figure shows
adsorption–desorption isotherms and the corresponding pore
size distribution plot for mesoporous Mn assemblies (Mn
MNAs) along with those of non-templated analogous. The
adsorption isotherm of the Mn MNAs sample is of type IV
with a combination of H and H -type hysteresis loop (according
to IUPAC classification), which is ascribed to an interconnected
3
N
2
the [010] direction) of tetragonal Mn
the SAED pattern, in Figure 2c, reveals that the Mn
3
O
4
, respectively. Besides,
NPs
3
O
4
3
O
4
3 4
O
embedded in the pore walls are randomly oriented, showing a
series of Debye-Scherrer diffraction rings that can be assigned
3
O
4
to the hausmannite structure of Mn
734).
Consistent with electron microscopy studies, the wide-angle
XRD pattern of the Mn MNAs material revealed its highly
3 4
O (JCPDS card no. 24-
2
3
0
[21]
mesoporous structure with slit-like pores.
Emmett–Teller (BET) surface area and total pore volume
The Brunauer–
3
O
4
crystalline nature as shown in Figure 2d. All of the diffraction
peaks can be readily indexed to the hausmannite phase of
2
assessed from the adsorption branch of the isotherms are 90 m
-1
3
-1
g
and 0.19 cm g , respectively. The mesoporous Mn
assemblies feature a narrow non-local density functional theory
NLDFT) pore-size distribution with the maximum of the peak
3 4
O
Mn
It is worth noting that the XRD pattern of Mn
identical to that of the precursor NPs, and no other diffraction
peaks were detected for impure crystalline phases (e.g., Mn
and MnO oxides). This is collaborated with Raman
spectroscopy results, which indicates only the characteristic
3
O
4
with a tetragonal lattice structure (space group: I4
1
/amd).
3
O
4
MNAs is
(
centered at ~6.6 nm (Figure 3, inset). This pore size is related to
the void space among the assembled NPs. Given an average
interparticle distance of 12.8 nm (determined by SAXS), the
difference between the repeat distance and the pore size gives
an estimate of the wall thickness at 6.2 nm. This value
corresponds well with the diameter of the Mn O NPs obtained
3 4
by TEM (ca. 6.7 nm) as well as SAXS analysis (ca. 6.9 nm).
Thus, it appears that the layers of interconnected NPs that
2
O
3
2
-1
shifts of hausmannite structure (647, 362, 304 and 281 cm ) for
the Mn MNAs, see Figure S2 in the Supporting Information.
Also, the broadening of the XRD peaks suggests that Mn
occurs as very fine grained particles. On the basis of Scherrer
3
O
4
3
O
4
[
20]
analysis
Mn
mesoporous Mn
Mn NPs. Taken together with TEM images, these results
verify that the constituting Mn NPs prevent, to a great extent,
of the (211) reflection, the average domain size of
3 4
comprise the walls of Mn O MNAs are single.
3
O crystallites was estimated to be ~7 nm for both
4
–
3
4 4
O assemblies and precursor (BF -capped)
3
O
4
3
O
4
interparticle sintering and maintain their initial nanostructure
during heating treatment.
o
Figure 3.
corresponding NLDFT pore-size distribution plots (inset) of the polymer-
templated mesoporous assemblies (Mn MNAs) (i) and random aggregates
Mn RNAs) (ii) of Mn NPs (STP: standard temperature and pressure).
N
2
adsorption–desorption isotherms at -196
C and the
3 4
O
(
3
O
4
3 4
O
Crucially, the polymer-templated ASA process yielded
porous network of assembled Mn NPs with mesoscale
porosity. For comparison, in the absence of template, the BF
capped Mn NPs are randomly aggregated into close-packed
structures. This is illustrated by the N adsorption-desorption
3
O
4
–
4
-
3
O
4
2
isotherms shown in Figure 3; the shape of which confirms the
difference in morphology between the templated and non-
Figure 2. a, b) Representative TEM images (inset of panel a: particle-size
distribution histogram showing an average particle diameter of 6.7 0.9 nm)
and high-resolution TEM image of an individual Mn O NP (inset of panel b),
3 4
templated NP assemblies. The
N
2
adsorption-desorption
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isotherms of the random aggregates of Mn
3
O
4
NPs (Mn
3
O
4
optimum catalyst loading for 1 oxidation was attained at 10
mol % (see Table 1, entries 7 and 8). At 50 C, the conversion to
o
RNAs) are of type IV with a H
found with random porous networks with slit-like pores.
Analysis of the adsorption data reveals that this sample has a
3
-type hysteresis loop, typically
[
21]
1a was moderate (ca. 46%, Table 1, entry 9), while upon
o
increasing the temperature at 70 C, 1 was selectivity oxidized to
2
-1
o
BET surface area of 107 m g and a total pore volume of 0.10
1a with 61% yield. Higher reaction temperature (90 C), however,
3
-1
cm g . In addition, the estimated NLDFT pore-size distribution
plot (assuming slit-shaped pores) indicates the formation of NP
aggregates with ca. 2.7 nm-sized interstitial voids. In order to
did not result in significant improvement of 1a formation (Table 1,
entry 10). Control experiments showed that in the absence of
catalyst or TBHP oxidant, no oxidation of 1 occurred (even over
a 4-hour reaction period), indicating that oxidation reaction is
really a catalytic process.
estimate the micropore volume in these materials, we performed
o
CO
2
physisorption measurements at 0 C and the obtained
isotherms were analyzed by the Dubinin-Radushkevich (DR)
method (see Figure S3 in the Supporting Information). The DR
Table 1. Conditions evaluation in the oxidation of styrene (1) using various
Mn O –based catalysts and oxidants.
3 4
[
a]
plot for the Mn
shows a micropore volume of 0.02 cm g (corresponding to a
microporosity of ~11%), while random aggregates of Mn NPs
3
O
4
assemblies prepared with polymer template
3
-1
3
O
4
3
-1
show a micropore volume of 0.05 cm g (corresponding to a
microporosity of ~50%). On the basis of the above results, it can
thus be concluded that the polymer-templated sample mainly
contains mesopores within the assembled structure, while the
randomly agglomerated NPs adopt a more densely packed
structure with significant fraction of micropores (d < 2 nm).
[
b]
Yield (%)
Conv.
(%)
Entry
Catalyst
Oxidant
TBHP
[b]
1
a
1b
1c
1d
Catalytic activity
To assess the catalytic activity of the Mn O MNAs and to
3 4
determine the optimum reaction conditions, the oxidations of
styrene (1) using different oxidants and solvents was initially
study. Following various catalytic experiments, an almost
quantitatively conversion of 1 (ca. 93%) was observed in the
[
c]
27%
4%
3%
4%
1
Mn
3
O
4
4
MNAs
38%
[c]
2
3
4
5
6
7
8
9
1
1
1
Mn
3
O
MNAs
MNAs
MNAs
MNAs
MNAs
MNAs
MNAs
MNAs
MNAs
NPs
TBHP
TBHP
TBHP
60%
88%
93%
38%
61%
50%
7%
9%
6%
8%
9%
Mn
3
O
4
7%
12%
24%
[
c]
Mn
3
Mn
3
Mn
3
Mn
3
Mn
3
Mn
3
Mn
3
Mn
3
Mn
3
Mn
3
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
10%
presence of 10 mol % (based on 1) Mn
equiv. of tert-butyl hydroperoxide (TBHP) in 2 mL of CH
3
O
4
MNAs catalyst and 3
CN,
[
d]
3
O
2
Nr
within 3 h (Table 1, entry 3). In all cases, the corresponding
benzaldehyde (1a) was observed as the major product
accompanying with small amounts of phenyl acetaldehyde (1b),
epoxystyrene (1c) and double oxidative product 2-oxo-2-
phenylacetaldehyde (1d), see Table 1. Noted that addition of
equimolar amount of TBHP (with respect to 1) resulted in
incomplete oxidation of 1, while in the presence of 2 equiv. of
TBHP, a 60 % conversion of 1 was observed in 3 h (Table 1,
entries 1 and 2). The increase of the TBHP amount (5 eq.) did
not result in a significant enhancement of the transformation of 1,
while the selectivity to 1a notably decreased (Table 1, entry 4).
H
O
2 2
<1%
64%
74%
70%
92%
53%
65%
45%
<1%
42%
49%
46%
62%
34%
40%
31%
[e]
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
5%
6%
8%
9%
4%
6%
3%
12%
14%
9%
5%
5%
7%
8%
3%
9%
5%
[
[
e]
f]
[
f]
0
1
2
13%
12%
9%
RNAs
bulk
3 4
Among the solvents studied, the catalytic performance of Mn O
MNAs can be improved when the reaction is carried out in 13
acetonitrile, affording 61% yield of 1a in 3 h; comparatively,
methanol and ethanol resulted in a lower oxidation activity,
giving 1a in 49% and 22% product yield, respectively, under
similar conditions (Figure S4, Supporting Information). This is
probably due to the efficient coordination of 1 to the surface
6%
[a] Reaction conditions: 1 (0.4 mmol), catalyst (0.04 mmol, 10 mol% with
respect to 1), TBHP (3 equiv.), CH CN (2 mL), for 3 h at 70 C. [b] Based on
3
the consumption of 1 determined by GC-MS. [c] These experiments were
performed with 1.0 equivalent (entry 1), 2.0 equivalent (entry 2) and 5.0
equivalent (entry 4) of TBHP. [d] No reaction. [e] These experiments were
performed with 0.02 mmol (entry 7) and 0.06 mmol (entry 8) of catalyst. [f]
o
active sites of Mn
appreciable oxidation of 1 takes place when molecular oxygen
1 atm) and hydrogen peroxide (H ) were used as oxidants
under the same conditions (Table 1, entries 5 and 6). Besides, in
the case of H oxidation (3 equiv.), a large amount of gas-
bubbles appeared as soon as the oxidant was injected into the
reactor, implying a fast catalytic decomposition of H by the
Mn NPs. The reaction activity also strongly depends on the
catalyst loading and reaction temperature. We found that the
3
O
4
in polar solvents. Meanwhile, no
o
o
These reactions were carried out at 50 C (entry 9) and 90 C (entry 10).
(
2 2
O
It is notably that Mn
than individual Mn
NPs. In particular, when BF
3
O
4
MNAs shows higher catalytic activity
NPs and random aggregates of Mn O
3
O
4
3 4
O
2
–
2
4
-capped Mn
3
O
4
NPs were
introduced as catalyst, they afforded a 1 conversion of 53% in 3
h (Table 1, entry 11), which is remarkably lower than that of
O MNAs. This may be attributed to the aggregation of NPs,
3 4
resulting in a decreased number of active sites available for
2
O
2
3
O
4
Mn
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catalysis. Also, random Mn
3
O
4
NP aggregates (Mn
3
O
4
RNAs),
The Mn
3
O
4
MNAs also demonstrated exceptionally high
although possess higher surface area, gave a lower conversion
of 1 (ca. 65%), under identical conditions (Table 1, entry 12).
Overall, these findings suggest that the high activity of the
stability under the examined conditions. The reusability of the
catalyst was assessed by performing repeated catalytic tests, in
which 1 was oxidized under the reaction conditions described
above. After each test, the catalyst was recovered from the
reaction mixture by filtration, washed several times with
deionized water and used for the next catalytic run. Catalytic
results, shown in Figure 4a, indicated that the conversion level
of 1 remains as high as 88–90% (at 3 h reaction time) after five
catalytic runs, reflecting excellent reusability. Characterization of
the regenerated catalyst by XRD showed that the phase
3 4 3 4
Mn O MNAs can be attributed to the small grain size of Mn O
particles, which provides a large number of surface active sites,
and the 3D mesoporous structure, which facilitates reactant
contact with the NP surface. In agreement to this, we also found
2
-1
that use of Mn
resulted in approximately two times lower activity than that found
for mesoporous assemblies of Mn NPs (see Table 1, entry
3). Notable, the catalytic activity of Mn MNAs compared
3 4
O micropowder (BET surface area < 1 m g )
3
O
4
1
3
O
4
remained Mn
nm, as determined by Scherrer analysis), Figure S5 of the
Supporting Information. However, physisorption
measurements revealed that the retrieved sample has a BET
3 4
O and the crystallite size did not change (ca. 7
higher or even comparable to that of other commonly used metal
oxide catalysts (see Table S1, Supporting Information). For
example, Mn O MNAs catalyst outperforms the catalytic activity
3 4
N
2
[
22]
[23]
[24]
[25]
[25]
2
-1
of MoO
3
, MoO
2
, Fe
3
O
4
, NiFe
2
O
4
and Mg0.4Fe2.6
O
4
surface area of 68 m g , which is lower than that of the fresh
[
26]
2
-1
3
-1
NPs, NiCoFe spinel-type oxide nanosheets , NiO and CoO
microparticles and mesoporous vanadium silicate-1 zeolites
while it nearly as effective as the Co
Zr-doped CeO
conditions.
catalyst (ca. 90 m g ), and a pore volume of 0.12 cm g . In
addition, pore size analysis points to the formation of a slightly
wider pore-size distribution with a peak maximum at 6.3 nm
(Figure S6 of the Supporting Information). These preliminary
studies indicate a structural deformation (i.e., small shrinkage of
[27]
[28]
,
[
29]
[30]
3
O
4
and CuO NPs and
[
31]
2
nanorods
studied under similar reaction
3 4
the pore structure) of the Mn O NP assemblies during catalysis.
To check the possibility of leaching Mn from catalyst, ICP-MS
analysis was performed on the solution following a 15-h reaction
time (after the catalyst was removed by centrifugation). The
results indicated that there is no leaching of Mn to the solution
phase; the metal content was below the detection limit of 0.3 ng
-
1
L .
To study the general applicability of the above oxidation
process, various para-substituted styrenes (2 and 3) and cyclic
alkenes (4 and 5) were examined. Figure 4b summarizes the
results obtained from these oxidation reactions using Mn
MNAs catalyst and TBHP oxidant. As can be seen, both methyl
2) and chloro (3) para-substituted styrenes were selectively
oxidized by the Mn MNAs, yielding the corresponding
aldehydes (2a and 3a) as the only products. However, oxidation
of electron-deficient aromatic styrene (p-Cl-substituted)
3 4
O
(
3
O
4
2
proceeded faster than electron-rich styrene 3 (p-Me-substituted),
giving the respective aldehyde in 98% and 90% yield,
respectively, in
3 h. Furthermore, an almost quantitative
consumption (ca. 95%) of cyclohexene (4) was observed by GC-
MS under the conditions described above, in which the
corresponding conjugated cyclohexenone (4a) was obtained at
90% relative yield with a small amount of the allylic cyclohexenol
(
4b, 5% yield) in 2 h. However, in contrast to cyclohexene,
cyclooctene (5) was oxidized to cyclooctene oxide (5a) and
cyclooctenone (5b) at 31% and 12% relative yields, respectively.
Noted that aliphatic 1-octene does not oxidized under the
present conditions (results not shown).
Regarding the mechanism of these organic transformations,
we believe that a radical process probably is involved in the
oxidation of alkenes to carbonyl compounds; although the
Figure 4. a) Recycling study of Mn
3
O
4
MNAs in the oxidation of styrene (1). b)
formation of
interaction of TBHP with the Mn
a
metal dioxirane-type intermediate due the
catalyst cannot be excluded.
Time profiles of para-substituted styrenes and cycloalkenes (2–5) oxidation
over Mn MNAs catalyst. Reaction conditions: 0.4 mmol of substrate, 0.04
mmol of catalyst, 3 eq. of TBHP, 2 mL of CH
3
O
4
3 4
O
o
3
CN, 70 C.
This hypothesis found support from above oxidation reactions in
which electron-rich aryl alkenes (styrenes) were oxidized faster
than cyclic and aliphatic alkenes. In addition, further evidence
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could be obtained by the oxidation of a series of aromatic
saturated hydrocarbons, such as ethyl benzene (6),
diphenylmethane (7), fluorene (8) and cumene (9). In general,
the oxidation of saturated -carbon C–H bonds in aromatic
alkanes, under mild oxidative conditions, represents one of the
cyclooctanone (10a) in 51% yield, accompanied with
cyclooctanol (10b) in low yield (6%) within a 3-h reaction period.
To the best of our knowledge, there is no report that describes
cyclooctane oxidation with such a high relative yield (89%) to
cyclooctanone using Mn-based catalyst. Overall, these findings
[
32]
most important and synthetically useful reactions.
Such
3 4
suggest that the present Mn O MNAs/TBHP catalytic system
organic transformations usually suffer from very low selectivity
and proceed under harsh reaction conditions, like high
temperature and pressures or high-energy radiation.
represents an expeditious approach towards a lab as well as a
large-scale production of carbonyl compounds from styrenes or
aromatic hydrocarbons in high yields and selectivity.
[
2]
Surprisingly, under the examined conditions, the aromatic
hydrocarbons 6–8 were selectively oxidized to the desired
carbonyl compounds (6a–8a) in appreciable yields, as shown in
Scheme 2. While very small amounts (~1–5% product yields) of
the corresponding secondary alcohols 6b–8b were detected
among the reaction mixture by GC-MS. This observation
indicates that the oxidation reaction proceeds through the initial
formation of the corresponding alcohols, which rapidly oxidized
to ketones 6a–8a. In order to support this assumption, we
examined the oxidation of 1-phenylethanol (6b) under the same
conditions. Indeed, within 3 h, acetophenone (6a) was obtained
as the only product in 91% yield (Scheme 2). In the case of
cumene (9), acetophenone (9a) was observed by GC-MS and
Conclusions
In summary, we have successfully prepared mesoporous
assemblies of Mn
through a polymer-assisted aggregating self-assembly method.
Structural characterization with SAXS, TEM, XRD and N
physisorption indicated that Mn MNAs material consists of an
interconnected network of 6–7 nm-sized hausmannite (Mn
NPs and exhibits a large surface area of 90 m g with a narrow
range of pore sizes (the peak maximum centered at about 6.6
3 4
nm). These mesoporous Mn O assemblies demonstrate great
3 4 3 4
O NPs (Mn O MNAs) with accessible pores
2
3
O
4
3
4
O )
2
-1
1
H NMR as the major product, accompanying with a significant
application perspectives in catalytic oxidation of various aromatic
amount of the corresponding tertiary alcohol (9b, ~9% product
yield). Acetophenone is a C–C bond cleavage product which is
formed from the corresponding peroxy- or alcoxy radical
and cyclic alkenes as well as aryl alkanes with tert-butyl
hydroperoxide. As compared to individual Mn
aggregated Mn NPs and bulk Mn
reactivity of Mn MNAs is attributed to the 3D open-pore
3
O
4
NPs, randomly
3
O
4
3 4
O solid, the higher
[33]
intermediate during the oxidation process.
3
O
4
structure and large accessible surface area, which allow facile
mass transport throughout the assembled structure. Moreover,
product analysis indicated that the reaction between the
3 4
selected alkenes and the TBHP–Mn O catalyst proceeds via an
allylic oxygenate intermediate obtained by a radical process;
however, further mechanistic studies are needed to fully
understand the mechanism of these reactions. These
preliminary results open up the possibility of using mesoporous
3 4
assembled structures of small-sized Mn O NPs for selective
alkene and aryl alkane oxidations.
Experimental Section
Chemicals
Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene
-
1
glycol) block copolymer (Pluronic P123, M
(
(
n
~ 5800 g mol ), oleic acid
90%), oleylamine (70%), N,N-dimethylformamide (DMF, 99.9%), xylene
>98%), acetonitrile (99.9%), n-hexane (95%), toluene (99.7%), absolute
Scheme 2. Oxo-functionalization of aromatic hydrocarbons 6–9 into the
corresponding carbonyl compounds catalyzed by Mn MNAs. In
parentheses: the corresponding isolated yield and selectivity. Reaction
ethanol and tert-butyl hydroperoxide solution (TBHP, 70% in water) were
obtained from Aldrich Chemical Co. Nitrosonium tetrafluoroborate
3 4
O
(
NOBF
4
, 97%) was purchased from Acros Organics. Manganese (II)
CO , 98%) was acquired from Sigma-Aldrich. All
conditions: 0.4 mmol substrate, 0.04 mmol catalyst, 3 equiv. TBHP, 2 mL
o
acetate (Mn(CH
3
2 2
)
CH
3
CN, 70 C.
organic substrates were of high purity and were obtained from Aldrich. All
chemicals were used as received without further purification.
3 4
Of particular note, Mn O MNAs catalyst also exhibited very
3 4
Synthesis of Mn O NPs
good catalytic activity and chemoselectivity in oxidation of
aliphatic cycloalkanes, such as cyclooctane (10), with TBHP.
Catalytic results showed that 10 oxidized to the corresponding
Mn
3 4
O NPs (6–7 nm in size) stabilized with oleic acid were prepared
[
34]
according to a literature procedure. In detail, 1 mmol of Mn(CH
3 2 2
CO )
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10.1002/cplu.201600460
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and 2 mmol oleic acid dissolved in 10 mmol of oleylamine were added in
pressure (P/P
o
) region. The total pore volumes were estimated from the
15 mL of xylene in air atmosphere. After slowly heating to 90 °C, 1 mL of
o
adsorbed amount at the P/P = 0.98 and the pore size distributions were
deionized water was injected into the solution under vigorous stirring, and
the resulting solution was aged at 90 °C for 3 h. A total of 100 mL of
ethanol was added to precipitate the NPs, which was followed by
centrifugation to retrieve the NPs in powdery form. The NPs synthesized
derived from the adsorption branch of the isotherms using the nonlocal
density functional theory (NLDFT) model (assuming slit-like pores). The
Raman spectra were collected at room temperature on a Nicolet Almega
XR micro-Raman spectrometer equipped with a 473 nm blue laser (15
mW) as an excitation source. Inductively coupled plasma-mass
spectrometry (ICP-MS) analysis was performed on a Perkin Elmer
NexION 300X instrument. For ICP-MS analysis, 1 mL of the sample
solution was evaporated to dryness, followed by the addition of 100 mL
-
were dispersed in n-hexane to form a stable colloidal solution (10 mg mL
1
).
3 4
Ligand-exchange of Mn O NPs
3
of 2% HNO .
The ligand-exchange process was carried out according to a previously
[
35]
Catalytic reactions
reported method. In a typical reaction, 5 mL of Mn
hexane was added in 5 mL of a solution of NOBF
and the mixture was kept stirring at room temperature until the NPs were
transferred to the DMF phase. The Mn NPs were then collected by
precipitation with toluene followed by centrifugation at 13000 rpm for 15
min. The ligand-stripped Mn NPs were dispersed in DMF to give a
final concentration of 240 mg mL .
O
3 4
NP dispersion in
4
in DMF (10 mg/mL)
In a typical experiment, a mixture of hydrocarbon (0.4 mmol) and catalyst
(0.04 mmol) in 2 mL of acetonitrile was placed in a 4 mL glass reactor
closed with a Teflon cap. The reaction was initiated by adding TBHP (3
eq.) and the mixture was vigorous stirred at 70 C. The reaction
conversion and the products characterization were realized by
3 4
O
o
O
3 4
-
1
a
combination of gas chromatography – mass spectrometry (Shimadzu
1
GCMS-QP2010 Ultra) and H NMR spectroscopy (Bruker AMX 300 MHz)
3 4
Synthesis of mesoporous Mn O MNAs
by withdrawing small aliquots from the reaction mixture. The
spectroscopic data of the products are in agreement with the
corresponding commercially available.
3 4 3 4
The mesoporous Mn O NP assemblies (Mn O MNAs) were synthesized
as follows: 0.2 g of Pluronic P123 (EO70PO20EO70) block copolymer was
dissolved in 1.5 mL of anhydrous ethanol at room temperature. Then, 0.5
3 4
mL of colloidal Mn O NP solution was slowly added dropwise to the
surfactant solution with continuous stirring. The resulting mixture was
kept at room temperature for 2 h and then stored in an oven at 40 C for
Acknowledgements
o
about 6 to 7 days under static condition. Removal of the template
This work was supported by the European Union and the Greek
Ministry of Education (NSRF) under the ERC Grant Schemes
(ERC-09, MIS 374071). We kindly thank Prof. S. Pergantis
(University of Crete) and Dr. A. Katsoulidis (University of
Liverpool) for collecting the ICP-MS and SAXS data,
respectively.
3 4
between the Mn O NPs as well as interparticle connection was achieved
o
-1
by heating the dry product at 350 C for 4 h in air, using a 0.5 C min
heating rate.
For comparison, random aggregates of Mn
also prepared by a similar procedure without the addition of polymer
template. Bulk-like Mn solid was also prepared by thermal
decomposition of MnCO
3 4 3 4
O NPs (Mn O RNAs) were
3
O
4
o
3
in air at 950 C for 2 h. Powder XRD confirms
3 4
the formation of single phase Mn O .
Keywords: manganese • mesoporous materials • nanoparticles
•
heterogeneous catalysis • alkenes
Physical characterization
Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer
o
Diamond system. Thermal analysis was conducted from 40 to 600 C in
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Entry for the Table of Contents
FULL PAPER
Mesoporous networks of cross-linked
E. Skliri, S. Papadogiorgakis, I. N.
6
–7 nm Mn
3
O
4
nanoparticles (NPs)
Lykakis, G. S. Armatas*
with large internal surface area and
uniform pores have been successfully
prepared via a polymer-assisted
aggregating self-assembly method.
These assembled Mn O NP
3 4
networks demonstrate great
application perspectives in catalytic
oxidation of various aromatic and
cyclic alkenes as well as aryl alkanes
with tert-butyl hydroperoxide.
Page No. – Page No.
3 4
Mesoporous Assembled Mn O
Nanoparticle Networks as Efficient
Catalysts for Selective Oxidation of
Alkenes and Aryl Alkanes
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